So I'm opening this up again, because its a valid issue.
The issue is that on gnome wayland there is no "main" bash/zsh/... shell from which the xserver is started so the bash profiles aren't read and enviroment variables aren't set.
Upstream fixed this in this commit: https://gitlab.gnome.org/GNOME/gnome-session/commit/7e307f8ddb91db5d4051c4c792519a660ba67f35
This obviously doesn't work on Solus, because there is no /etc/profile but a /usr/share/etc/profile and so the login shell never gets run. I tried to manually fix that and see if it works afterwards, but unfortunally there seems to be another issue somewhere.
The obvious user facing issue this creates is that /snap/bin isn't added to the $PATH and /var/lib/snapd/desktop isn't added to $XDG_DATA_DIRS, so Snaps just don't show up in Gnome. The other issue is that $QT_QPA_PLATFORMTHEME is not set to gtk2, so Qt apps (eg. KeepassXC,...) isn't using *any* styling at all.
The thing is that flatpak is added to path and working fine, because it is in /usr/share/gdm/env.d/... *but* its overwriting the XDG_DATA_DIRS variable there, if you comment that line out flatpak is broken but snaps are there.